This invention is directed to a ferrous alloy for use as a knife in wood chipping apparatus. More particularly, the invention relates to a heat treated alloy steel, which when formed into knives is especially adaptable for rotary wood chippers. Though suitable as knives for soft and hard wood chippers, the further description of the alloy of this invention will be directed to its use in hardwood chippers. As a consequence, the demands on the knives used therein will be better appreciated.
Hardwoods, such as oak, walnut, cherry, maple and ash, are processed for chipping by first sizing the logs to lengths of four to six feet and diameters up to 21 inches. After debarking and washing, the logs are conveyed to the chipper, where, in a matter of a few seconds, the logs are converted into chips. A hardwood chipper may typically use fifteen (15) knives, radially arranged in a drum rotating at from 2000-3000 rpm.
The life of a chipper knife, whose radial edge comprises the cutting or chipping portion of the knife, varies considerably. The knives may be worn or dulled through normal chipping, damaged through contact with foreign objects such as nails or knots in the wood, or affected by chipping frozen logs from wood cut in winter.
While the wood chipper described above has been the traditional device for creating wood chips, recent years have seen the popularization of portable "tree harvesters." These devices are moved to the logging area where it chips logs without recourse to debarking or trimming to length. The chips are fed into a waiting truck and then taken to the pulp mill for processing. These portable chippers do not possess the same degree of knife support as provided in the larger stationary chipper. Hence, knife failures due to gross fracture occur much more frequently in the portable units compared to experience with knives in stationary units. The knives in the portable unit are the same size as in the stationary units but only three are used.
Heretofore, to meet the harsh demands of chipper knives, the grade of steel used therefor comprised, by weight,
______________________________________ C .45-.50% Mn .20-.40% Si .80-1.0% Cr 8.0-9.0% Mo 1.20-1.50% W 1.0-1.40% V .20-.40% Fe balance ______________________________________
For use as knives, such steel is quenched and tempered to 56/58 HRC, i.e. austenitized at 1850.degree. F. (1010.degree. C.), oil quenched, and double tempered at 975.degree. F. (524.degree. C.).
The most important characteristics of chipper knives are edge retention and toughness. Other significant considerations are ease of heat treatment, machinability, and dimensional stability. However, attainment of these desirable attributes are commercially meaningless if cost is not competitive. For the first time, the alloy steel of this invention brings together all such attributes, and at a competitive cost. As a consequence, the alloy steel of this invention is superior to the current grade now being used for chipper knives.